This invention relates generally to nondestructive testing, and more particularly to using an eddy current probe and methods of assembling the same.
Eddy current (EC) inspection devices are used to detect abnormal indications in a component being tested such as, but not limited to, gas turbine engine components. For example, known EC inspection devices may be used to detect cracks, pings, dings, raised material, and/or other surface imperfections on a surface of the component, and/or to evaluate material properties of the component including the conductivity, density, and/or degrees of heat treatment of the component.
During operation, known EC devices measure the interaction between an electromagnetic field generated by the EC device and the component being tested. For example, at least some known EC devices include a probe coil that generates a magnetic field. When the coil is positioned adjacent to a conductive component, an eddy current is generated on the surface of the component. A flaw on and/or near the surface of the component disrupts the eddy current field causing a secondary field to be produced that is received by the eddy current probe coil or by a sensor coil in the eddy current probe. The altered secondary magnetic field is converted to an electrical signal that may be recorded, for example, on a strip chart recorder.
In use, a substantially constant pressure is applied to the EC probe as the coil moves along the surface of the component being tested. The constant pressure facilitates maintaining an integrity of the signal generated by the EC probe. However, when the EC probe is not oriented substantially perpendicular to the surface of the component being tested, a “lift-off effect” may be created.
To facilitate reducing lift-off effects, at least one known EC probe includes a dual-coil probe, e.g. a differential probe, that includes a pair of coils with an opposite polarity. Each coil in the dual-coil probe generates an electrical signal when the probe contacts a surface of the component being tested. More specifically, when the dual coil probe passes over a smooth surface of the component being tested, the signals cancel each other. However, when the dual coil probe passes over a local physical abnormality on the surface, the probe generates a signal that is proportional to the size, depth, etc., of the physical abnormality.
FIG. 1 is a front view of an exemplary known eddy current (EC) probe 500. Eddy current probe 500 includes two differential coils coupled to a solid surface of eddy current probe 500. FIG. 2 is a top view of eddy current probe 500 shown in FIG. 1. FIG. 3 is a front view of eddy current probe 500, and illustrates the lift-off effect in an indexing direction. Eddy current probe 500 is unable to conform to the surface of the article being tested. FIG. 4 is a front view of eddy current probe 500 and illustrates the lift-off effect in a scan direction. Known differential probes may only be tiltable up to approximately 2° before the EC signal deteriorates.
As shown in FIG. 1, when a non-continuous component surface feature is inspected, such as a feature on a rotating part, known differential probes may have difficulty testing sharp curvatures, in such areas as corners and cusps. During operation, when such probes encounter a corner or cusp, the differential probe device may become skewed to the surface of the component, such that a resulting lift-off effect may cause a loss of usable data. Accordingly, known EC devices may be less effective in generating an accurate response when the EC device is used to detect conditions on a component having complex geometries, and/or a component having irregular conditions, such as may be prevalent in components including sharp indexing or objects that extend into the path of the probe such that the probe cannot consistently remain normal to the scan surface. Known EC devices include multiple EC probes oriented in an array, such that a larger area of a surface is tested at once. However, arrays of this type can increase the lift-off effect on non linear surfaces.